/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.http;

import org.springframework.lang.Nullable;
import org.springframework.util.MultiValueMap;
import org.springframework.util.ObjectUtils;

/**
 * Represents an HTTP request or response entity, consisting of headers and body.
 *
 * <p>Typically used in combination with the {@link org.springframework.web.client.RestTemplate},
 * like so:
 * <pre class="code">
 * HttpHeaders headers = new HttpHeaders();
 * headers.setContentType(MediaType.TEXT_PLAIN);
 * HttpEntity&lt;String&gt; entity = new HttpEntity&lt;String&gt;(helloWorld, headers);
 * URI location = template.postForLocation("https://example.com", entity);
 * </pre>
 * or
 * <pre class="code">
 * HttpEntity&lt;String&gt; entity = template.getForEntity("https://example.com", String.class);
 * String body = entity.getBody();
 * MediaType contentType = entity.getHeaders().getContentType();
 * </pre>
 * Can also be used in Spring MVC, as a return value from a @Controller method:
 * <pre class="code">
 * &#64;RequestMapping("/handle")
 * public HttpEntity&lt;String&gt; handle() {
 *   HttpHeaders responseHeaders = new HttpHeaders();
 *   responseHeaders.set("MyResponseHeader", "MyValue");
 *   return new HttpEntity&lt;String&gt;("Hello World", responseHeaders);
 * }
 * </pre>
 *
 * http实体,主要存放数据:
 * <ol>
 *     <li>头信息,headers{@link HttpHeaders}</li>
 *     <li>数据信息,Body</li>
 * </ol>
 * 该类主要应用与 web 项目做返回值
 * @param <T> the body type
 * @author Arjen Poutsma
 * @author Juergen Hoeller
 * @see org.springframework.web.client.RestTemplate
 * @see #getBody()
 * @see #getHeaders()
 * @since 3.0.2
 */
public class HttpEntity<T> {

    /**
     * The empty {@code HttpEntity}, with no body or headers.
     */
    public static final HttpEntity<?> EMPTY = new HttpEntity<>();


    private final HttpHeaders headers;

    @Nullable
    private final T body;


    /**
     * Create a new, empty {@code HttpEntity}.
     */
    protected HttpEntity() {
        this(null, null);
    }

    /**
     * Create a new {@code HttpEntity} with the given body and no headers.
     *
     * @param body the entity body
     */
    public HttpEntity(T body) {
        this(body, null);
    }

    /**
     * Create a new {@code HttpEntity} with the given headers and no body.
     *
     * @param headers the entity headers
     */
    public HttpEntity(MultiValueMap<String, String> headers) {
        this(null, headers);
    }

    /**
     * Create a new {@code HttpEntity} with the given body and headers.
     *
     * @param body    the entity body
     * @param headers the entity headers
     */
    public HttpEntity(@Nullable T body, @Nullable MultiValueMap<String, String> headers) {
        this.body = body;
        HttpHeaders tempHeaders = new HttpHeaders();
        if (headers != null) {
            tempHeaders.putAll(headers);
        }
        this.headers = HttpHeaders.readOnlyHttpHeaders(tempHeaders);
    }


    /**
     * Returns the headers of this entity.
     */
    public HttpHeaders getHeaders() {
        return this.headers;
    }

    /**
     * Returns the body of this entity.
     */
    @Nullable
    public T getBody() {
        return this.body;
    }

    /**
     * Indicates whether this entity has a body.
     */
    public boolean hasBody() {
        return (this.body != null);
    }


    @Override
    public boolean equals(@Nullable Object other) {
        if (this == other) {
            return true;
        }
        if (other == null || other.getClass() != getClass()) {
            return false;
        }
        HttpEntity<?> otherEntity = (HttpEntity<?>) other;
        return (ObjectUtils.nullSafeEquals(this.headers, otherEntity.headers) &&
                ObjectUtils.nullSafeEquals(this.body, otherEntity.body));
    }

    @Override
    public int hashCode() {
        return (ObjectUtils.nullSafeHashCode(this.headers) * 29 + ObjectUtils.nullSafeHashCode(this.body));
    }

    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder("<");
        if (this.body != null) {
            builder.append(this.body);
            builder.append(',');
        }
        builder.append(this.headers);
        builder.append('>');
        return builder.toString();
    }

}
